1. Field of the Invention
The present invention relates to a power amplifier used mainly in various kinds of radio equipment, communication equipment, measuring devices, and the like.
2. Description of Related Art
FIG. 1 shows the circuit configuration of a conventional amplifier disclosed in, for example, JP 2004-153378. This amplifier includes an RF operation section 1 and a DC bias circuit 2.
The RF operation section 1 includes an RF operation transistor 3 for carrying out voltage-to-current conversion, an impedance 4, a DC blocking capacitor 5, and a feedback inductor 15. The emitter of the RF operation transistor 3 is grounded via the feedback inductor 15, and the base of the RF operation transistor 3 is connected via the DC blocking capacitor 5 to an RF signal input terminal 6 to which a high frequency signal is inputted. A voltage is supplied from a power supply 7 to the collector of the RF operation transistor 3 via the impedance 4. A high frequency signal output can be obtained from the collector of the RF operation transistor 3, and the collector is connected to an RF signal output terminal 8.
The DC bias circuit 2 includes a base current compensation transistor 9, a power supply 10, a reference current source 11, a current reference transistor 12, a feedback resistor 13, and an RF blocking resistor 14. The base of the RF operation transistor 3 is connected to the emitter of the base current compensation transistor 9 via the RF blocking resistor 14. Further, the base of the RF operation transistor 3 also is connected to the base of the current reference transistor 12. The RF operation transistor 3 forms a current mirror circuit together with the current reference transistor 12.
The emitter of the current reference transistor 12 is grounded via the feedback resistor 13. The collector of the current reference transistor 12 is connected to the power supply 10 via the reference current source 11.
The separate power supplies, i.e., the power supply 10 and the power supply 7, are illustrated in FIG. 1. However, the power supplies are not necessarily provided separately, and a single power supply may be used instead of using the power supply 10 and the power supply 7. Further, the feedback inductor 15 is not necessarily required, and the emitter of the RF operation transistor 3 may be grounded directly.
Hereinafter, an operation of the amplifier configured as described above will be described. The RF operation transistor 3 is biased by the current reference transistor 12 having a diode structure in which the base and the collector are short-circuited. The base current compensation transistor 9 is inserted so as to form a current mirror in order to prevent a current variation due to the absolute variation in DC current amplification factor (hFE) between the RF operation transistor 3 and the current reference transistor 12. A backflow of a high frequency signal is prevented by the RF blocking resistor 14. The linearity of the amplifier can be enhanced by sufficiently increasing the resistance value of the feedback resistor 13.
However, in the above-described configuration, two stages of transistors of diode connection are configured in series. Hence, when a general SiGe process is considered, a power supply voltage of at least 1.6 V is needed since the voltage between the emitter and the base is about 0.8 V. This is inconvenient when a variation in power supply voltage and various kinds of dispersions are taken into consideration.